Bob Quick, Britain's anti-terrorism chief, quit as a policeman after being
told he had lost the confidence of MI5 and the Home Secretary because of his
security blunder.

Mr Quick admitted that he could have compromised a huge police and intelligence operation of co-ordinated raids on a suspected al-Qaeda cell.

The Assistant Commissioner was photographed entering Downing Street carrying a secret briefing note on which details of the undercover operation – codenamed Pathway – could be seen.

After months of undercover surveillance, officers were forced to launch the raids at short notice, and a former Scotland Yard anti-terrorism chief said that the blunder could have "risked lives". One suspect was arrested by armed police outside a busy university library in Liverpool.

Mr Quick tendered his resignation to Boris Johnson, the London mayor and chairman of the Metropolitan police authority, who announced it live on radio, to the surprise of Sir Paul Stephenson, the Commissioner, and Jacqui Smith, Home Secretary.

He also announced that John Yates, an Assistant Commissioner, will replace him - becoming the third anti-terrorism chief in three years as an embattled Scotland Yard continues to suffer from upheaval.

Mr Quick met with Jacqui Smith at the Home Office and was told that he had lost her support and that of the security services. The Home Secretary then met Sir Paul to discuss the matter.

Mr Johnson told BBC Radio 4's Today programme that he did not pressure Mr Quick to quit his £168,000-a-year role, and said that the counter terrorism head decided to leave of his own volition.

He said: "In the end Bob Quick decided it was the best thing to do. It's matter of sadness and he had a very very distinguished career in counter terrorism."

He continued: "I want to stress there was no effort to get him out."

Home Secretary Jacqui Smith said Mr Quick felt his position was "untenable" following the publication of the photographs and thanked him for his work.

She said: "Sir Paul Stephenson has informed me that Assistant Commissioner Bob Quick has offered his resignation following the publication of certain photographs yesterday.

"Although the operation was successful he felt that his position was untenable.

"I want to offer my sincere appreciation for all the outstanding work he has done in this role which has helped keep this country safe."

Chris Grayling, the shadow home secretary, told Sky News: "I think it was the right thing to do ... Bob Quick made his position completely untenable, not just within the organisation but in the eyes of the public.

"The buck has to stop somewhere. This was a serious breach, which could have jeopardised, and indeed may have impacted upon an investigation into a possible terror threat.

"I cannot understand why a document of this kind was not in a locked briefcase as a matter of course."

Scotland Yard chief Sir Paul paid tribute to Mr Quick, calling him "a tremendous police officer who has served with dedication and professionalism throughout his career".

He said: "He has accepted that he made a serious error and that has led to his resignation this morning. I would like to thank Bob for the excellent job he has done leading the national response to the terrorist threat.

"It is important in such a critical policing role to announce Bob's successor. I have today appointed Assistant Commissioner John Yates as head of Specialist Operations."

The departure of Mr Quick has left officers at Scotland Yard in shell-shock and a lack of experience at the top of anti-terrorism police ranks.

Two of the country’s seasoned anti-terrorism officers, Peter Clarke and Andy Hayman, have retired in recent years and Mr Quick's number two, Deputy Assistant Commissioner John McDowall, has been struck down by a serious and debilitating illness.

The alleged terror cell based in the North West was thought to have been plotting an attack in Britain.

Sources said the men discussed targeting nightclubs and shopping centres, thought to have included Manchester’s Trafford Centre and Arndale Centre.

Twelve men were arrested in Manchester, Liverpool and Clitheroe, Lancs. One man was British-born, the rest were Pakistanis staying on student visas. They ranged in age from a teenager to a 41-year-old man.

At least one suspect was held by armed officers in open view at Liverpool John Moores University, while another was shot with a stun gun.

Police said the arrests were part of an “ongoing investigation” and were searching premises for bomb-making equipment.

The mastermind of the alleged terrorist cell was believed to have been Rashid Rauf, an al-Qaeda suspect who was implicated in several other plots. He was reported to have been killed in a US drone attack in Pakistan last year.

Mr Quick, who is in overall charge of Britain’s anti-terrorism strategy, had gone to Downing Street to brief the Home Secretary on the latest developments in what had been a top secret operation.

The briefing note showed details of the locations and manner of the intended arrests by “dynamic entry — firearms”. It also showed where the suspects would have been held and the names of the six senior officers in charge of the operation.

Shortly after lunchtime, newspapers and broadcasters were contacted by the D-Notice Committee in an attempt to prevent the picture being published, though Mr Quick’s actions had caused such alarm the committee was initially able to tell editors only that they “might be in possession” of a photograph that compromised national security, without saying what it was.

It was not long before the operation had to take place.

Eight addresses were searched. Two men were arrested on Galsworthy Avenue, Cheetham Hill, Manchester; two were held at an internet cafe on Cheetham Hill Road, three were held in Cedar Grove, Liverpool; one at Liverpool John Moores University; one on Earle Road, Liverpool; two at a Homebase store in Clitheroe in a raid involving up to 100 officers; and one when a white van was stopped on the M602 between Liverpool and Manchester.

Students at the university were warned of the incident. One, Nicholas Higgins, said: “There was an announcement [on a loudspeaker] asking all students to stay away from the windows. They told us that there were terrorists outside and to keep away from the windows because they had a bomb on them.”

In Cheetham Hill, residents said the people staying at the house in Galsworthy Avenue had been there for only a few months.